1. Field of the Invention
The present invention relates to an electrically rewritable nonvolatile semiconductor memory device and a system therefor, for example, a NAND flash memory or a NAND flash memory and a controller therefor.
2. Description of the Related Art
In recent years, as nonvolatile semiconductor memory devices which can electrically rewrite data, NAND flash memories are widely used in a variety of apparatuses. The NAND flash memory includes a plurality of memory cells (referred to as NAND cells hereinafter) connected in series and selection gate transistors arranged at both ends of the NAND cells.
In the NAND flash memory, in a read operation, a memory cell having a threshold value close to a read voltage disadvantageously has a random error in which data may be correctly read or erroneously read.
Jpn. Pat. Appln. KOKAI Publication No. 6-83716 discloses the following device. Three pieces of identical data are prepared by a data generating circuit and redundantly written to three different memory cells, respectively. In a read operation, the data are read from the memory cells, and data determined by majority decision by using a majority logic circuit are output as read data.